AIRBORNE RADIOCESIUM IN SCOTS PINE AND NORWAY SPRUCE NEEDLES

This paper presents the activity of radiocesium (Cs-134 and Cs-137) and chemical composition of Scots pine and Norway spruce needles in southern Finland during the years 1987-1991. In 1991, content and vertical distribution of radiocesium in soil have been determined both under and between canopies. The total Cs-134 content per square meter in soil layers varied within the range of 470-9500 Bq/m2 in stands of Norway spruce and 1200-2300 Bq/m2 in stands of Scots pine; the corresponding figures for Cs-137 were 5000-88 000 and 11 100-28 600 Bq/m2. About 80-90% of the radiocesium was located in the humus and in the first 3 cm of mineral soil. There were no statiscally significant differences in the contents of Cs-134 and Cs-134 in the soil under and between the tree crowns. Most of the Cs-137 which originated from nuclear weapons fallout, was in the top mineral soil layers. The radiocesium concentrations in the needles varied considerably from stand to stand. In 1987, with the exception of a few forests, the radiocesium concentrations in the 2-year-old (C + 1) needles were higher than those in the current (C) needles. In the other years, the situation was the opposite. The regression between Cs-137 concentration in current needles and the year was linear for pine needles and polynomial for spruce needles.